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| Titel |
A model of the methane cycle, permafrost, and hydrology of the Siberian continental margin |
| VerfasserIn |
D. Archer |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 10 ; Nr. 12, no. 10 (2015-05-21), S.2953-2974 |
| Datensatznummer |
250117941
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| Publikation (Nr.) |
 copernicus.org/bg-12-2953-2015.pdf |
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| Zusammenfassung |
A two-dimensional model of a sediment column, with Darcy fluid flow,
biological and thermal methane production, and permafrost and methane
hydrate formation, is subjected to glacial–interglacial cycles in sea
level, alternately exposing the continental shelf to the cold atmosphere
during glacial times and immersing it in the ocean in interglacial times. The
glacial cycles are followed by a "long-tail" 100 kyr warming due
to fossil fuel combustion.
The salinity of the sediment column in the interior of the shelf can be
decreased by hydrological forcing to depths well below sea level when the
sediment is exposed to the atmosphere. There is no analogous advective
seawater-injecting mechanism upon resubmergence, only slower diffusive
mechanisms. This hydrological ratchet is consistent with the existence of
freshwater beneath the sea floor on continental shelves around the world,
left over from the last glacial period.
The salt content of the sediment column affects the relative proportions of
the solid and fluid H2O-containing phases, but in the permafrost zone
the salinity in the pore fluid brine is a function of temperature only,
controlled by equilibrium with ice. Ice can tolerate a higher salinity in
the pore fluid than methane hydrate can at low pressure and temperature,
excluding methane hydrate from thermodynamic stability in the permafrost
zone. The implication is that any methane hydrate existing today will be
insulated from anthropogenic climate change by hundreds of meters of
sediment, resulting in a response time of thousands of years.
The strongest impact of the glacial–interglacial cycles on the atmospheric
methane flux is due to bubbles dissolving in the ocean when sea level is
high. When sea level is low and the sediment surface is exposed to the
atmosphere, the atmospheric flux is sensitive to whether permafrost inhibits
bubble migration in the model. If it does, the atmospheric flux is highest
during the glaciating, sea level regression (soil-freezing) part of the
cycle rather than during deglacial transgression (warming and thawing).
The atmospheric flux response to a warming climate is small, relative to the
rest of the methane sources to the atmosphere in the global budget, because
of the ongoing flooding of the continental shelf. The increased methane flux
due to ocean warming could be completely counteracted by a sea level rise of
tens of meters on millennial timescales due to the loss of ice sheets,
decreasing the efficiency of bubble transit through the water column. The
model results give no indication of a mechanism by which methane emissions
from the Siberian continental shelf could have a significant impact on the
near-term evolution of Earth's climate, but on millennial timescales the
release of carbon from hydrate and permafrost could contribute significantly
to the fossil fuel carbon burden in the atmosphere–ocean–terrestrial
carbon cycle. |
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